1. Field of the Invention
The present invention relates to a method and an apparatus for molding a ceramic sheet used, for example, for a laminated exhaust gas sensor and a laminated heater.
2. Description of the Related Art
The doctor blade method (casting method) and the extrusion molding method are known for molding a ceramic sheet.
Generally, a ceramic sheet is molded by the doctor blade method. In this method, ceramic powder is mixed with an organic binder, a solvent, etc. to produce a slurry. The slurry is injected into a dam having a doctor blade arranged on a carrier film, and the carrier film is moved at a predetermined speed in a predetermined direction. As a result, the slurry flows out continuously from the gap between the doctor blade and the carrier film. After that, the slurry is dried together with the carrier film and separated from the carrier film thereby to produce a ceramic sheet of a predetermined thickness.
The doctor blade method, however, requires the use of a slurry with a large amount of solvent added thereto, and therefore a large number of pores are formed in the dried sheet after volatilization of the solvent. The presence of a large number of pores causes such inconveniences as a reduction in the ceramic powder filling factor, an increased burning shrinkage and variations of the burning shrinkage. This poses the problem of variations of the product size.
It is very difficult to maintain the thickness of a slurry. Therefore, a thick sheet cannot be molded from a slurry. When manufacturing a product using a thick sheet, therefore, the problem is posed that an appropriate number of thin sheets are required to be stacked.
Also, it is very difficult to acquire monodisperse ceramic powder having a uniform grain size distribution. Therefore, the grain size distribution of the ceramic powder has a certain margin. Even if a thick sheet could be molded by the doctor blade method, ceramics of larger grains naturally sediment faster in the slurry in the drying process. Thus, a density difference occurs between the upper surface portion and the lower surface portion of the sheet. As a result, a difference of the burning shrinkage occurs between the upper and lower surfaces, thereby posing the problem of a warped product. This problem may be caused also with a thin sheet processed by the doctor blade method.
In the extrusion molding method, on the other hand, the filling factor of ceramic powder is so high that a thick sheet can be molded.
The extrusion molding is of two types, plunger and screw (auger). The extrusion molding of a plunger type is a method in which a ceramic material is filled in a mold and extruded from the mold by piston, and can produce a predetermined fluidity depending on the manner in which the ceramic material is filled. Nevertheless, the disadvantage of this method is that the ceramic material cannot be extruded continuously.
The extrusion molding of screw type, on the other hand, is a method in which the ceramic material is continuously extruded from a mold by rotating a screw. Due to the variations of fluidity of the ceramic material in the screw extruder, however, an attempt to mold a wide, thin sheet using an extruder having a screw of a small diameter would partially increase or decrease the molding pressure, resulting in an irregular flow of the ceramic material which in turn leads to wrinkles in the sheet.
Various solutions to this problem have been proposed as described below.
(1) The pressure exerted on the ceramic material is sufficiently equalized by increasing the screw diameter. This solution can prevent the generation of wrinkles on the sheet but leads to a very bulky screw extruder. As a result, when changing the ceramic material, the disassembly and cleaning process requires a great number of steps. Also, since a greater amount of ceramic materials are left in the apparatus, the yield of the material is considerably deteriorated.
(2) Japanese Unexamined Patent Publication No. 63-307903 discloses a technique for the plunger type, in which the flow rate of the sheet is substantially equalized at the ends and the central portion of the sheet by setting the temperature higher at the ends than at the central portion of the sheet. For the screw type of extrusion molding, however, unlike the plunger version, the flow rate at the central portion is not always high, and therefore the flow rate adjustment is impossible in the case where the flow rate at the ends or a given portion is higher. Therefore, this technique is not directly applicable to the screw type of extrusion molding. Even if applicable, many defects would be caused in the sheet in the case where the screw diameter is large.
(3) Japanese Unexamined Patent Publication No. 61-125805 proposes a technique for regulating the flow rate by extending or retracting a rectification block. However, this is intended for applications to thick, wide sheets and fails to achieve the object of the present invention.
(4) Japanese Unexamined Patent Publications No. 9-328366 and No. 10-152379 propose a technique in which the fluidity of the ceramic material (body) can be improved by changing the plasticizer or the like added to the body to produce a uniform sheet. This method, however, poses the problem that a change in the composition of the additive changes the various ceramic characteristics including the burning shrinkage resulting in different product performance.
In spite of the various techniques thus far proposed as solutions to prevent the wrinkling of a ceramic sheet as described above, an effective solution for the screw extruder has yet to be discovered. Especially, a method has not yet been established for the extrusion molding of a wide, thin ceramic sheet in the screw extruder which can suppress wrinkling.
The present invention has been developed in view of the problem points of the prior art described above, and the object thereof is to provide a method and an apparatus for molding a ceramic sheet, in which a comparatively wide, thin ceramic sheet can be extrusion molded while suppressing the wrinkling using a screw extruder of a small diameter.
According to a first aspect of the invention, there is provided a method of molding a ceramic sheet using a molding apparatus having an extruder of screw type and a mold arranged at the forward end portion of the extruder, wherein the ceramic material introduced into the extruder is molded into a sheet by extrusion from the mold, and wherein the ceramic material passing through the mold is divided into a plurality of transverse areas, for each of which the temperature is regulated in the process of extrusion molding.
What is most noticeable about this aspect of the invention is that the ceramic material passing through the mold is extrusion molded while regulating the temperature of each of a plurality of transverse areas into which the mold is divided.
The mold is preferably divided into three or more transverse areas. As a result, at least the central portion can be temperature-regulated separately from the side portions. A specific number of areas into which the mold is to be divided can be appropriately selected in accordance with the width, etc. of the ceramic sheet to be molded.
The functions and effects of this aspect of the invention will be explained below.
In this aspect of the invention, the ceramic material being passed through a mold is extrusion molded while regulating the temperature of a plurality of transverse areas into which the mold is divided. As a result, the transverse difference of the molding rate and the resulting geometrical deformation of the ceramic sheet can be accurately suppressed.
In the case where the central area of the ceramic material passing through the mold is corrugated (wrinkled) due to a higher molding rate, for example, the temperature of the central area is kept relatively low. Specifically, the temperature of the central area is reduced and/or the temperature of the other areas is increased.
In this way, the temperature of the area corresponding to the transverse central portion of the ceramic material passing through the mold is relatively decreased and so is the fluidity thereof. This phenomenon occurs due to the correlation between the fluidity and the temperature of the ceramic material. The relative decrease of fluidity in the central area of the ceramic material leads to the relative decrease of the extrusion molding rate in the particular area. As a result, the ceramic sheet is extruded out of the mold at a substantially uniform extrusion molding rate in transverse direction, thereby improving the shape of the ceramic sheet free of wrinkles.
Even in the case where a wrinkle is not generated in the central area but in other portions, the relative decrease of the temperature of the ceramic material passing through the mold in the area corresponding to the particular portions can cause the relative decrease of the fluidity and the relative decrease of the extrusion molding rate at the particular area. As a result, the extrusion molding of the ceramic sheet extruded from the mold is corrected to a transversely uniform rate, with a shape improved to be free of wrinkles.
According to this aspect of the invention, the shape can be positively corrected as described above. Even a comparatively wide, thin ceramic sheet which has conventionally failed to be successfully extrusion molded and wrinkled in the screw extruder can be molded smoothly to a very excellent shape.
Thus, according to this aspect of the invention, there is provided a method of molding a ceramic sheet, in which even a comparatively wide, thin ceramic sheet can be extrusion molded while suppressing the wrinkling in a screw extruder.
According to a second aspect of the invention, there is provided a method for molding a ceramic sheet, in which the correlation data on the molding rate of ceramic sheet to be extrusion molded is obtained by measurement for the portion corresponding to each area, and the temperature is preferably regulated based on the correlation data on the molding rate thus obtained. In this way, the temperature can be regulated automatically even in the case where the condition of the ceramic material extruded into the mold from the extruder is liable to change, thereby making it possible to control the shape of the ceramic sheet more accurately. The correlation data on the molding rate for each area described above may be the molding rate data obtained by a non-contact speed sensor or may be the geometrical data or the displacement data correlated with the molding rate. This is by reason of the fact that the difference in molding rate is reflected in the sheet shape by wrinkles and corrugations. Therefore, the measurement of the shape and displacement can replace the molding rate data.
According to a third aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder and the width W of the ceramic sheet preferably hold the relation Wxe2x89xa73d. Specifically, a thin ceramic sheet having a width W more than 3d is liable to wrinkle. According to this aspect of the invention, the superior functions and effects described above are exhibited and the wrinkling or the like can be prevented.
According to a fourth aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder and the width W of the ceramic sheet may hold the relation Wxe2x89xa75d. Although wrinkles are more easily developed in this case, the superior functions and effects described above can be exhibited positively and thus the wrinkling can be prevented.
According to a fifth aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder is preferably not more than 70 mm. In this case, the whole screw extruder can be built in compact form, and the disassembly work for replacing parts or materials can be performed by a single worker. Thus, the molding process can be simplified and the number of molding steps can be reduced. On the other hand, the smaller the outer diameter of the screw of the screw extruder, the more difficult it is to produce a wide ceramic sheet. According to this invention, however, the functions and effects described above can be exhibited, and therefore even a wide ceramic sheet which has conventionally been difficult to produce can be molded in a superior shape.
Also, by reducing the screw diameter to 70 mm or less, the internal volume of the screw extruder can also be reduced. As a result, the amount of air introduced into the screw extruder can be reduced, so that air is prevented from mixing with the ceramic sheet produced for an improved product quality. The prevention of air from mixing in the ceramic sheet can also suppress the internal defects of the ceramic sheet. Further, an insulation failure or cracking can be prevented in electrical applications of the ceramic sheet as an electrical insulating material.
According to a sixth aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the thickness of the ceramic sheet is preferably not more than 1.5 mm. The width of the ceramic sheet having a thickness not more than 1.5 mm cannot conventionally be increased as wrinkles would otherwise develop. According to this aspect of the invention, on the other hand, the functions and effects can be exhibited considerably even with the thickness of not more than 1.5 mm, and even a wide ceramic sheet which has conventionally been difficult to produce can be molded in a superior shape.
According to a seventh aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the thickness of the ceramic sheet may not be more than 300 xcexcm. In this case, wrinkles are more liable to develop. Nevertheless, the molding in an excellent shape is made possible by the superior functions and effects exhibited as described above.
According to an eighth aspect of the invention, there is provided a method for molding a ceramic sheet, wherein the mold includes a plurality of retractable rectification plates arranged to change the flow resistance. The extrusion molding can thus be carried out while adjusting the flow resistance of the ceramic material by extending/retracting the rectification plate while at the same time regulating the temperature. In this case, wrinkles or the like irregular shapes can be corrected more effectively by controlling the physical flow resistance by extension/retraction of the rectification plates in addition to the temperature regulation for each area.
According to a ninth aspect of the invention, there is provided an apparatus for molding a ceramic sheet, comprising a screw type of extruder and a mold arranged at the forward end of the extruder for extrusion molding a ceramic material introduced into the extruder to form a ceramic sheet, wherein the mold includes means for regulating the temperature of the portion of the ceramic sheet corresponding to each one of the transverse areas into which the mold is divided.
What is most noticeable about this aspect of the invention is that the temperature regulation means for regulating the temperature of the ceramic material is arranged in each of the areas.
The temperature regulation means can employ any of various methods as described later. The mold is preferably divided into at least three transverse areas to make it possible to control at least the central portion and the end portions.
Also, the temperature regulation means can be arranged on one or both of the upper and lower dies of the mold. In the case where the temperature regulation means is arranged on both the upper and lower dies, the mold can be divided transversely in the same number or different numbers of areas for both the upper and lower dies.
Now, the functions and effects of the molding apparatus according to this aspect of the invention will be explained.
The mold of the molding apparatus according to this aspect of the invention has the temperature regulation means as described above. In the case where a ceramic sheet is extrusion molded using this molding apparatus, therefore, the ceramic material passing through the mold can be extrusion molded while regulating the temperature thereof by the temperature regulation means for each of a plurality of portions of the ceramic sheet corresponding to the transverse areas into which the mold is divided. As a result, a very excellent molding method can be positively implemented.
In this aspect of the invention, there is provided a ceramic sheet molding apparatus in which a comparatively wide, thin ceramic sheet can thus be extrusion molded by the screw extruder while suppressing the wrinkling.
According to a tenth aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the temperature regulation means preferably includes a chamber associated with each of the areas into which the mold is transversely divided, and a heating medium circulation means included in each chamber for circulating a heating medium to heat or cool the material. In this case, the temperature of the ceramic material in each area can be easily regulated by controlling the flow rate or the temperature of the heating medium circulated in each chamber.
According to an 11th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the temperature regulation means can include a heater controllable for each of the areas into which the mold is divided transversely. In this case, the temperature of the ceramic material in the respective areas can be increased separately from each other by differentiating the heating capacity of each heater. Also, the heater can be used with the heating medium circulated in the chamber, in which case the temperature can be regulated easily by any combination of various heating or cooling factors, thereby improving the temperature control accuracy.
According to a 12th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, preferably comprising a molding rate correlation data measuring means for measuring, for each corresponding area, the molding rate of the ceramic sheet extruded from the mold, and control means for controlling the temperature regulation means based on the molding rate correlation data acquired from the molding rate correlation data measuring means. In this case, the temperature regulation means can be controlled accurately in accordance with the molding rate distribution fed back from the molding rate correlation data measuring means.
The molding rate correlation data measuring means is not confined to a speed sensor for measuring the molding rate directly, but may be an indirect measuring means such as a shape sensor, a displacement sensor, etc. which is controlled to measure the shape or displacement correlated with the molding rate.
According to a 13th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder and the width W of the ceramic sheet preferably hold the relation Wxe2x89xa73d. In this case, as in the cases described above, the effect of preventing the wrinkling can be exhibited especially conspicuously.
According to a 14th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder and the width W of the ceramic sheet may alternatively hold the relation Wxe2x89xa75d. In this case, too, as in the cases described above, the effect of preventing the wrinkling can be exhibited conspicuously.
According to a 15th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the outer diameter d of the screw built in the extruder is preferably not more than 70 mm. In this case, as in the cases described above, the whole screw extruder can be reduced in size to a comparatively compact form, and the disassembly work including the job of replacing the materials can be carried out by a single worker. Thus, the process can be rationalized and the number of the production steps can be reduced, while at the same time effectively preventing wrinkling, etc. Also, as described above, the intrusion of air into the ceramic sheet produced can be suppressed.
According to a 16th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the thickness of the ceramic sheet is preferably not more than 1.5 mm. In producing a ceramic sheet having a thickness of not more than 1.5 mm, the width thereof cannot be increased sufficiently as wrinkles would otherwise be caused. In spite of this, the functions and effects described above can be exhibited conspicuously in this range according to this invention.
According to a 17th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the thickness of the ceramic sheet is preferably not more than 300 xcexcm. In this case, as described above, the ceramic sheet is more liable to wrinkle. Nevertheless, the aforementioned functions and effects makes it possible to mold the ceramic material in an excellent shape.
According to an 18th aspect of the invention, there is provided an apparatus for molding a ceramic sheet, wherein the mold preferably includes a plurality of rectification plates arranged in retractable manner for changing the flow resistance of the ceramic material. In this case, the shape of the ceramic sheet can be corrected even more effectively by controlling both the temperature regulation means and the rectification plates.